The present invention relates to a multifrequency sender receiver in a multi-time slot digital data stream for use with a telephone system operating in time frames each consisting of a plurality of time slots.
Pulse code modulated (PCM) digital signals in a telephone system enable a multiplicity of conversations to be transmitted over a two wire digitally multiplexed line commonly known as a T1 trunk line. The quantized PCM data representative of a sample of a particular portion of a conversation to be sent over a telephone trunk line between a calling and called party is serially interleaved or multiplexed with a number of other conversations into channels within a discrete time period. If 8 binary bits were to represent each quantized value, then up to 256 (2.sup.8) discrete values could represent the particular sample. The T1 trunk line comprises 24 multiplexed channels serially presented to a telephone switching system, with each conversation sampled at a frequency of 8 kilohertz (a period of 125 microseconds). Each channel on the T1 line therefore is allocated a time slot of approximately 5.2 microseconds. Several T1 lines grouped together are further multiplexed into what is known as a Line Group. It is possible to switch a conversation on one channel or time slot of one line group to another channel or time slot of the same or another line group through a Time Slot Interchanger.
In addition to PCM coding of voice communications, dial pulsing and multifrequency (MF) pulsing representing calling and called numbers between telephone trunks are known in the art. MF signaling is particularly adaptable to PCM switching techniques because of the speed and accuracy over conventional dial pulse (DP) techniques. MF signals are digitally encoded in PCM format to represent valid number information over telephone trunks by various combinations of 2 of 6 frequencies (700, 900, 1100, 1300, 1500, 1700 Hz) in the voice band.
In order to generate a multifrequency tone, prior art systems have utilized a memory containing in PCM format a representation of one of the tone pairs for a particular digit. Multifrequency tones are particularly adaptable for tone generation because each multifrequency tone can be represented by 21 PCM samples to accurately represent the particular digit. This is because of waveform symmetry for each multifrequency tone pair. All even quarter periods of the waveform are mirror images of the corresponding odd quarter periods. Also, every half quarter period of the waveform is inverted. The tones are read out of the multifrequency tone memory cyclically by a 21 state up/down counter. The outputs of the memory are appropriately complemented and inverted by necessary gating circuitry, depending upon the particular quarter period of the waveform.
However, the requirement of a separate memory for representing each of the digits 0-9 and the other signals, such as KP and ST, necessarily increases the cost of the system over what the cost would be if fewer memories could be utilized.
A telephone system incorporating multifrequency signaling techniques will utilize multifrequency senders to send tone signals in time slots through the system and multifrequency receivers to receive the detect MF tone signals identifying calling and called numbers.
Depending upon traffic conditions, it would be desirable for the telephone system to be capable of allocating the multifrequency senders and receivers to specified time slots in which multifrequency tones are to be sent or received.
In addition, it is desirable to provide a capability of generating the required multifrequency tone utilizing fewer memories than are used in the prior art, thus decreasing the cost of the system.